1. Field of the Invention
The present invention relates to a power supply apparatus, and especially relates to a variable switching frequency power supply apparatus.
2. Description of the Related Art
A power supply apparatus is a very common electronic apparatus. The power supply apparatus supplies power to an external electronic apparatus to drive the external electronic apparatus. Therefore, the power supply apparatus is very important.
A transistor switch electrically connected to a transformer is controlled with a fixed frequency by a pulse width modulation controller of a related art power supply apparatus. Therefore, the root mean square value current and the peak current are larger when the input voltage is low. Low current resistance power components cannot be used. A lot of electric energy becomes thermal energy. The transformer enters the saturation state easily to lost inductance characteristics.
Therefore, the efficiency of the related art power supply apparatus is not good when the input voltage is low.